Chromium plating



"Patented July 7, 1925-.

UNITED'S'TATES" PATENT m Hanna 0. PIERCE AND drum n. numrnams, or xoxonco. INDIANA, ASSIGNORB no i -JAMES 0mm: Panama, 01* xoxomo, mourn.

cnaonimm rmrme.

No Drawing. 4

To all whom it may concern.

lie it knowuthat we, (1) IIIARRIIE C.

I Kokomo, (22) Kokomo, in the counties of invention the aging referre and smooth adherent deposlts are obtained square foot.

a process of electroplating chromium and to a bath or solution for use in connection therewith.

The electrolyte or bath most commonly used and described in the literature for the electrode os'ition of chromium in the form of smooti adherent deposits is the so-called .Sargents solution, an essential characteristic of which is that it contains chromic acid and a relatively small. amount of chromic sulfate. The best results are obtained with this solution after 'it has been aged for a time by the passage of an-electrie current through it. and even then the results are erratic and undesirable at current densities less than about 40 amperes per square foot. Higher current densities in'the neighborhood of 100 amperes per square centi-. meter "ordinarily are recommended.

In accordance with the rocess of our to is avoided at current densities as low as 14 ampcres per Our invention consistsgenerally in preparing a bath containing essentially chromic acid, ammonium sulfate and hydra-ted chromic hydroxide dissolved preferably in distilled'water and electrolyzing this solution usin .-either a chromium or other suitable ano e-such as iron or lead and a steel, brass or other metal cathode upon which the chromium deposit is desired and a current density ranging from about 14 up to about 125' amperes persquare foot.

The bath may be prepared in a variety of; ways for instance by dissolving chromic acid 1n water, preferably distilled, and'mixing with this solution a solution prepared by dissolving chromic sulfate and a quantity of ammonium hydroxide suflicient to approximately quantitatively decompose the chromic sulfate in water. An example of.

"and hydrated chromic hydroxide with or a preferred composition and the preferred directly as soon as it is prepared and it also the amount 'ust sufiicient to react with the Application and my 5, m4. .Berlal' No. 111,260.

method of preparing it in accordance with our invention-1s as follows: A-solution is prepared by dissolving-2 ounds of chromicacid 0. P. in gallon 0 Water, preferably distilled. A second solution isprepared'by dlssolving ounce of chromic sulfate in- 11; gallon of 'water and adding thereto to cc. of a 28% ammonium h droxide solution. The second solution is mixed into the first giving a plating solution which is ready for use directly without agin The density of the plating solution about 20.

A solution so repared has some of the characteristics of the so-called Sargents' solution after the latter has been aged, particularly in its appearance, being of a very dark color and opaque. It does not,- however, require aging, and is ready for use differs from anaged S-argents solution in that from it light, continuous, coherent and homogenous deposits apparently free from green chromium oxide can be'produced at current densities as low" as 1.4 amperes per square foot whereas in order 'to secure sunilar results with an agedj.Sa'rgents solution'-a current density of 40 amperes-or more per square foot .is necessary.

The composition of our plating bath may be varied within aflrather wide range, the range of variation bein somewhat greater than in the case of the .Sargents solution. The chromic acid content of the. bath-may vary from 80 to 450 grams per'liten The quantity of chromic sulfate used in making up the bath preferably amounts to from 1 to 2% of the chromic acid content. The preferred amountof ammonium hydroxide is chromic sul ate .or only slightly in. excess of that amount. The effect of the am-. monium hydroxide appears to be analogous to' that produced by aging, this effect. being secured 1n adegree proportional to the relative quantity used'up to. a quantity equivalent to the chromic sulfate. Consequently it is within the scope of our invention to use any substantial quantity of ammonium h droxide less than the chemical e uivalent of the chromic sulfate used for t 'e urpose of securin a corresponding effect. r consideringlt 10 solution of our invention as a solution of chromic acid, ammonium sulfate without chromic sulfate, it is within the scope of our invention to vary the quantities of ammonium sulfate and chromic sulfate,

the former from aquantity chemically equivalent to the hydrated chromic hydroxide content of the bath, down to the smallestsubstantial quantity capable of producing a beneficial effect, and the latter in inverse ratio.

The best temperature of operation is at about room temperature and should not ordinarily exceed 80 F.

As is stated above, either metallic chr'omium or an inert anode maybe employed but chromiumis preferred since its use tends to maintain the composition of the bath. The composition of the bath is not, however, completely maintained by the use of a chromium anode and it is necessary in order to maintain the composition of the bath to add to it chromic acid in quantity sufficient, as

' may be determined by analysis, to maintain from as low as 14 amperes per square the chromium content thereof.

The current density employed in the electrodeposition, as has been stated, may iary oot to-125 amperes or more per square foot, good deposits being obtained throughout this en- 'tire' range. For commercial electro-plating for protective and ornamental purposes the lower current densities, say from 14 to40 amperes'per square foot, are preferred since within this range it is possible to continuously produce smooth, continuous, adherent deposits of a good white color; With the lower current densities the current eificiency is low, but due to the nature of the deposits, they being continuous, coherent,'dense and hard, very thin deposits are sufficient. A deposit .00015 inchesor less thick such as. maybe produced in less than an hour with a current density of 25 amperes per square foot constitutes a commercial coating for "many with our invention of from .00005 to .0002

finishes on steel, lron, brass .and 'copper. Chromium coatings produced in accordance inchesin thickness compare favorably with standard nickelcoatings. which are from about .0003 to .0006 inches thick. 'The chromium coatings are highly protective due to their hardness and to the fact that they are. more continuous (have less pin holes) than other common coatings, for instance nickel coatings.

mo'spheric corrosion, nitric and sulfuric acids and to atests} involving the alternate wetting and drying of the coatings for a period of several months. The coatings have a very high luster,.-they can be polished by means of a buff wheel and colored and theyshow no tendency to peel or crack.

The color bf the deposits varies s mewhat Coating .0002 inches thick have beenfound to be-resistant to acid fumes, at-

with the current density employed in producing them from a bright tin white at low current densities to a dark gray color at high "current densities.

We claim:

1. A bath for the electrodeposition of chromium containing chromic acid and relatively small proportions of hydrated chro- .mi'c hydroxide and ammonium sulfate in aqueoussolution.

2. As a bath for the electrodeposition of chromium, an aqueous-solution of the reaction products of chromic acid, chromic sulfate, and ammonium hydroxide.

3. As a bath for the electrodeposit'ion'of chromium, an aqueous solution containing from 80 to 450 grams of chromic acid per liter,'and the reaction products of chromic sulfate in quantity amounting to from 1 to drated chromic hydroxide and ammonium sulfate. I

6. Process as defined in claim 5 in which the anode is metallic chromium.

7. Process as 'definedin claim 5 in which the composition of the bath is maintainedby the addition of chromic acid.

8. Process as defined in claim 5 in which a current density of from 14 to 125 amperes per square foot is employed 9. Process'as' defined in claim 5in which a current density of 14 to 40 amperes per I square footis employed.

10. Process of electrodeposltlng chromium.

which comprises passing an electric current at a current density of from 14 to 40 amperes per square foot through a bath containing'from 80 to 450 grams of chromic acid .per liter and the reaction products of chromic sulfate in quantity amounting to from 1 to 2%. by weight of the chromic acid content 'of the bath and a chemically equivalent quantity of ammonium hydroxide maintained ata temperature not over 80 F. between an anode ofimetallic chromium and a cathode of a metal to be electro-plated.

In testimony whereof, we aflix our signa tures. i

HARRIE c. PIERCE; CHAD r1. HUMPHRIES. 

